The spectral composition of X-rays provided by an X-ray tube depends on the acceleration voltage of the electron beam. The energy of the X-ray quantum increases with the accelerating voltage. Since all different sorts of tissues in a living body have different absorption properties depending on the energy of the X-rays impinging on the relevant tissue, this effect can be used to differentiate between different tissue compositions and thus allows more specific diagnosis of various pathological situations, e.g. tumors, kidney stones, or plaque deposition in blood vessels.
In computed tomography systems, an imaging apparatus is rotating around the body of interest, while new picture frames are taken after small angular displacements. Each frame is taken within a predefined time window which depends mainly on the resolution of the detector and rotational speed. By mechanical constraints and electronic complexity all time windows have the same duration during an individual examination. If several frames are taken at different energy levels, the type of tissue can be examined in addition to the general achievement of 3D pictures. For example, every other frame could be taken at a different energy level.
In order to achieve best separation between the spectra and an optimal image reconstruction, the supply voltage of the x-ray tube should be kept constant during the exposure time of the respective frame. If different energy levels are following short after each other, e.g. every other frame, it is also necessary to keep the transition between the different voltage levels as short as possible, or in case of examining a living human to prevent radiation from the x-ray tube during the transition. Radiation at intermediate levels deteriorates the image quality and imposes unused radiation dose on a patient and is generally not useful for achieving a high quality image.
The efficiency sensitivity to the CT system is much higher at higher electron beam energy and vice versa. Therefore by sufficient heating the emission current of the cathode has to be chosen high enough, that the low energy image frame is sampled at sufficient signal-to-noise ratio within a given time frame (e.g. 100 μs). The consequence is that an image taken at the higher acceleration voltage is prone to overexposure, due to the high sensitivity at higher voltage. In order to avoid this, the emission current has to be reduced again by reducing the heating. This process requires some tens to hundreds of milliseconds, which is obviously to slow for a frame-by-frame switching of the energy levels. Therefore this is typically achieved by shortening the exposure time at the higher voltage. While the exposure at low voltage extends over the full time window, the exposure at high voltage level covers only a fraction of it by switching the x-ray tube on and off with a grid-electrode within a few microseconds.
It is another requirement that the working pulse, i.e. the period with x-ray radiation and exposure of the detector, has to occur in the mid of the exposure time window, because of geometrical reasons,
It is further a requirement to achieve most freedom of choice of the sequence of high-voltage to low-voltage frame captures, i.e. any combination of a number of frame registrations at one voltage may be followed by any other number of registrations at some other voltage, e.g. the ratio of frame captures at two voltage levels Vn and Vm equals a ratio of n:m, with n and m being integer numbers.
It is an additional requirement, that the predefined time window for capturing a single image frame can be adjusted from a minimum value, e.g. 100 μs, to longer values, e.g. 500 μs, e.g. when the rotating speed of the CT gantry is reduced.
It is also a requirement that the means for achieving a dual- or multi-spectrum x-ray beam are small, lightweight and efficient.
A rather simple method to cover the field of use with dual energy examination is to make a helical scan with one energy level first in one direction and then travel back the same path with another energy level. This method is known as “back-to-back-scanning”. It is clearly apparent that there is a rather large time delay between these two scans in different directions, which may lead to a misregistration if the position of the body of interest or the organs has changed in between these two scanning steps.
It is known to apply different energy levels every full revolution, which method is known as “alternating scan”. If the organ of interest is rapidly moving, e.g. in case of a heart, there may still be misregistrations with this scan method.
It is also known to control the high voltage supply for altering the accelerating voltage of the X-ray tube by modulating a high voltage generator, which may be done with a time constant in the order of one ms. In contrast to that, the time difference between subsequent frames at a high rotation rate can be as small as 100 μs, which cannot be produced by current generator concepts.
WO 2010/015960 A1 shows an X-ray system comprising a radiation source, a high voltage generator and a modulation wave generator generating a modulation voltage wave having non-zero amplitude, which is combined with and modulates the source voltage between at least two different voltages.
US20100098217A1 shows a method to boost the output voltage of a high voltage DC power source by reconfiguring a chain of capacitors from a parallel to a series connection by means of multitude of controlled switches and diodes. There, the voltage during the boost phase cannot be fully controlled anymore as the extra energy is solely withdrawn from the series connection of capacitors. Moreover, the circuit consists of a multitude of elements that have to be operated and controlled at a very high voltage level. After returning to the lower voltage level, recharging of partially discharged capacitors produce settling effects of the high DC voltage leading to deterioration of the spectral quality of the x-ray beam. While it is simply possible to extent registration at the lower voltage, this cannot be achieved at the higher voltage due to the discharge of the capacitors.
It is also known to build a high voltage source for an X-ray tube with a modulated voltage by connecting a high voltage DC-source in series with a pulse transformer. In the EP817546A1 an X-ray system capable of quickly varying energy levels is shown comprising several different configurations of at least one high-voltage DC source in series with at least one transformer, the latter being supplied by a waveform generator. The transformer is connected to a secondary capacitor in parallel to the winding and forms a resonant circuit. The waveform generator generates a periodical waveform of a frequency close to the resonance frequency of the resonant circuit. Although it is intended to supply the waveform generator with arbitrary waveforms, including square wave pulse-shaped, it must be noted, that the influence of the resonant circuit behavior does not permit achievement of square wave or other pulse waveforms with a flat top property.
Another problem is saturation of the transformer which will occur after short time, due to the DC-nature of the secondary current. This will happen also, if it is attempted to register several image frames at the same (higher or the lower) voltage. Then the maximum possible voltage integral of the transformer will be exceeded. If saturation occurs the desired output voltage can no longer be maintained.